1. Field of the Invention
The present invention relates to a wiring board for an inkjet head, a method of polarizing piezoelectric elements, and an inkjet recording apparatus, and more particularly to a wiring board for an inkjet head, a method of polarizing piezoelectric elements, and an inkjet recording apparatus, which are suitable for polarizing piezoelectric elements.
2. Description of the Related Art
FIGS. 9A and 9B are respectively a side view and a plan view of the principal part of a head block comprising a conventional wiring board for an inkjet head.
As shown in FIG. 9B, a wiring pattern for installing upper surface electrodes (individual electrodes) of piezoelectric elements 2 and a drive IC 3, which is a drive element for applying a drive voltage between the electrodes of the piezoelectric elements 2, are formed on a wiring board 1, such as a flexible wiring board.
The lower surface of the wiring board 1 which includes lower surface electrodes of the piezoelectric elements 2 is bonded to a conductive plate (vibration plate) 4, by inserting a thermosetting adhesive, an anisotropic conductive adhesive, or an anisotropic conductive film, between the respective bonding surfaces and then bonding them together by applying heat and pressure. Furthermore, reflow soldering is used to bond the wiring board 1 with the upper surface electrodes (individual electrodes) of the piezoelectric elements 2 and to install the drive IC 3 onto the wiring board 1.
A plate 4 forming a common electrode for the respective piezoelectric elements 2 is electrically connected by soldering to a ground wire 1A of the wiring board 1, via a conductive material or wiring (not illustrated) leading from the lower surface to the upper surface of the piezoelectric element, or the like. Furthermore, in FIG. 9B, numeral 1B denotes a ground wire of the drive IC 3 and GND denotes a common ground terminal of the ground wires 1A and 1B.
The piezoelectric elements have a Curie temperature at which they lose their polarized state. This Curie temperature is different for each type of piezoelectric element.
If heat exceeding the Curie temperature is applied to a piezoelectric element 2, when bonding the piezoelectric elements 2 with the plate 4, bonding the wiring board 1 with the piezoelectric elements 2, or installing the drive IC 3 on the wiring board 1, then the piezoelectric element 2 becomes depolarized and is no longer able to function as an actuator.
Possible solutions for this include a method using piezoelectric elements having a high Curie temperature, or a method where the piezoelectric elements are polarized or re-polarized after being installed on the wiring board.
The former method is described in paragraph “0002” of Japanese Patent Application Publication No. 2003-55045. More specifically, paragraph “0002” states that “If the Curie temperature is high, then the piezoelectric elements have high heat tolerance, and degradation of piezoelectric characteristics due to heat can be restricted, to a relative degree, even if the piezoelectric elements are heated to a high temperature of approximately 200° C. when passed through a solder reflow oven during installation, for example.”
On the other hand, if the latter method is adopted, it is necessary to polarize the piezoelectric elements by applying a voltage exceeding the normal operating voltage range to the piezoelectric elements. Therefore, a high voltage is applied to the piezoelectric elements after they have been installed on the wiring board and before the drive IC is installed, in such a manner that a voltage exceeding the voltage tolerance is not applied to the drive IC.
More specifically, as shown in FIG. 10A, after installing a piezoelectric element 2 on the wiring board 1, the piezoelectric element 2 is polarized or re-polarized by applying a polarizing voltage E1 between the terminal IC on the wiring board 1, which is electrically connected to the individual electrode 2A of the piezoelectric element 2, and the ground terminal GND of the wiring board 1 which is electrically connected to the common electrode 2B of the piezoelectric element 2.
A drive IC is then installed on the wiring board 1, as shown in FIG. 10B. In order to drive the piezoelectric element 2, a signal from a drive signal source S is input to the signal input terminal Is of the wiring board 1.
If piezoelectric elements having a high Curie temperature (for example, a Curie temperature of 600° C.–700° C.) as described in Japanese Patent Application Publication No. 2003-55045 are used in order to prevent loss of polarization of the piezoelectric elements during installation on the wiring board, then this reduces the possible range of selection of the piezoelectric elements.
On the other hand, if the piezoelectric elements are polarized or re-polarized after being installed on the wiring board and the drive IC is then installed on the wiring board, then two installation processes are required. Moreover, as shown in FIG. 9A and FIG. 9B, it is desirable that the piezoelectric elements 2 and the drive IC 3 are installed as closely together as possible, but in this case, the heat used during installation of the drive IC 3 also affects the polarized or re-polarized piezoelectric elements 2.